There has heretofore been developed a tank type filter utilizing a disposable filter media belt, as disclosed in U.S. Pat. Nos. 3,087,620; 3,091,336; 3,221,885; and 4,396,505.
In this type of filter, a drag conveyor is circulated through the tank interior, typically across the tank bottom over a perforate plate covering a vacuum box, which receives filtered liquid which has passed through the portion of the filter media belt overlying the perforate plate.
According to the concept of this type of filter, the filter media belt is disposable, and is fed off a feed roll and thence through the tank interior, over the perforate bottom plate, and out of the tank to be collected for disposal. The filter media belt is periodically indexed as the section over the perforate bottom plate becomes clogged, drawing in an additional length from the feed roll at one end of the tank, while a corresponding used section of filter media is passed out from the opposite exit end wall of the tank.
The drag conveyor is employed to carry the heavy filtered solids out of the tank, and for this purpose has a series of spaced bars or "flights" extending across the width of the tank, connecting opposite chain loops disposed on either side thereof. The chain loops are driven by suitable sprockets during indexing to advance the conveyor through the tank interior. The filter media belt lies between the conveyor flights and the tank interior surfaces extending along the circulation path of the conveyor.
Since the media belt is not recirculated, as is the drag conveyor, the conveyor and filter media belt are not connected together, allowing the filter media belt to pass off the conveyor as each is indexed to move out of the tank, enabling the used segments of the filter media belt to be readily collected and disposed of. This unconnected relationship between the conveyor and filter media belt requires some means for driving the filter media belt to be advanced with the conveyor during indexing.
The disposable media belt is constructed of relatively weak material, such as thin filter paper and cannot be simply pulled through the tank, as by a powered windup roller, since the high friction created by the filter media belt lying beneath the flights of the chain conveyor develops a high resistance to belt movement relative to the conveyor, resulting in tearing of the filter media belt if it were simply attempted to be pulled through the tank. Tearing allows unfiltered liquid to pass through the openings and into the outlet.
It is also important that the filter media belt remain flat at all times, since bunching results in uncontrolled movement and in unfiltered liquid getting around the filter media belt edges and into the outlet.
According to past practices with such filters, frictional engagement of the conveyor flights is primarily relied on to move the filter media belt during indexing, although take up or pinch rollers are sometimes used to assist this action. The aforementioned rollers sometimes themselves create problems by the used filter media sticking to the rollers, causing wadding and bunching of the media on these elements.
Heretofore, the weight of the conveyor has been relied on to establish sufficient frictional engagement between the flights and the filter media belt. This arrangement creates a distributed driving engagement which operates very advantageously to advance the relatively low tensile strength media belt without bunching or tearing.
The simple weight biased frictional engagement between the conveyor and filter media requires that a relatively shallowly angled exit end wall be incorporated in the filter tank, and a maximum of 45.degree. of inclination has been considered to be the upper limit of such inclination. Any steeper angles reduce the component of the weight of the conveyor pressing on the filter media belt to a point where the frictional force developed is inadequate for achieving reliable advance of the belt by the movement of the conveyor. If slippage does occur, there is a tendency for solid particles to tear openings in the media belt and for the belt to bunch and wad.
The shallowly angled exit end wall adds substantially to the overall length of the filter apparatus.
This increase in overall length of the apparatus required by the shallowly inclined exit end wall is disadvantageous, as it requires considerable additional space to accommodate the filter, which space may not be available at a given site. This added length also directly increases the cost of the filter itself by the additional conveyor and tank structure components required to extend the filter length.
There has heretofore been employed in such filters chip shielding plates spaced above each conveyor chain loop. If a considerable wadding of the media belt occurs, due to excessive belt slippage, a positive drive is in effect established by the media wad wedging against a conveyor flight eliminating the slippage. However, this causes highly localized pulling of the media belt, and sometimes results in tearing of the filter media belt.